This invention relates to utilization of size-limited crystals of Na.sub.2 SO.sub.4.10H.sub.2 O (Glauber's salt) as a liquid-solid phase change material in storage of thermal energy.
The invention is particularly applicable to the utilization of Glauber's salt as the solid form of a L-S P-C material in a thermal energy storage device, which imparts interparticle motion to crystals in the chemical system comprising Na.sub.2 SO.sub.4 (sodium sulfate), water, and Glauber's salt.
Methods are known for storing thermal energy in and retrieving thermal energy from a liquid-solid phase-change (L-S P-C) material, for which sodium sulfate decahydrate is the solid form, wherein such material is maintained in a container and fluid is circulated over the outer surface thereof to effectuate heat exchange between the fluid and the L-S P-C material. A method and device of the foregoing types are described by Herrick in U.S. Pat. No. 4,154,292, which is incorporated herein by reference. As described therein with reference to the device (referred to therein as a "heat exchange device"), a liquid-solid phase-change material is sealed in a container, which is slowly rotated about a generally horizontal axis at a substantially constant rotational speed.
A problem encountered with utilization of Glauber's salt in the cycled storage of thermal energy is that the latter salt physically encapsulates Na.sub.2 SO.sub.4 (sodium sulfate) during the freezing mode, i.e., during removal of thermal energy or heat from the chemical system. As used herein, the term "cycled storage" means the cycle comprising the steps of introducing thermal energy into and removal thereof from a L-S P-C material. In the chemical system comprising Na.sub.2 SO.sub.4, H.sub.2 O and Na.sub.2 SO.sub.4.10H.sub.2 O the resulting encapsulation by Glauber's salt of Na.sub.2 SO.sub.4 decreases the rate of dissolution thereof in the liquid phase. Such dissolution is required for continuation of formation of Glauber's salt crystals from the Na.sub.2 SO.sub.4 and water components of the system. Thus, the aforementioned dissolution of Na.sub.2 SO.sub.4 is a prerequisite to maximizing the amount of Glauber's salt crystals which can be formed from (and concomitant release of heat for) a given amount of Na.sub.2 SO.sub.4.
The rate of requisite dissolution of the encapsulated Na.sub.2 SO.sub. 4 decreases with increasing thickness of the encapsulating Glauber's salt wall and increases with decreasing thickness thereof. As a corollary, the resistance to diffusion or other movement of Na.sub.2 SO.sub.4 through the encapsulating wall to the surrounding solubilizing aqueous liquid increases with increasing wall thickness and decreases with decreasing thickness thereof. As an overall result, such encapsulation decreases the rate of crystallization and the rate of heat release.
One approach to overcoming the foregoing problem is described by Herrick in U.S. patent application Ser. No. 706,895, filed Feb. 23, 1976, now U.S. Pat. No. 4,209,312 and incorporated herein by reference. Briefly stated, that approach includes addition of ferric ions with the goal of preventing the growth of large crystals.
It has now been found, by practice of the present invention, that the average size of Glauber's salt crystals formed by crystallization of Na.sub.2 SO.sub.4 from aqueous solution in systems comprising Na.sub.2 SO.sub.4, H.sub.2 O and Glauber's salt is decreased by including a fluorine-containing surfactant in the system. It has further now been found that decreasing the average size of such crystals in this manner advantageously results in decreasing the average thickness of the Na.sub.2 SO.sub.4 -encapsulating wall of Glauber's salt, ultimately increasing the rate, extent, or both rate and extent of Glauber's salt crystal formation.